Subscriber access provided by UNIV OF LOUISIANA
Fossil Fuels
Investigation into the ash deposits in a coal-fired traveling grate boiler with the ammonia present in the flue gas Xiaolu Chen, Zhiyuan Liang, Wenjun Yang, and Qinxin Zhao Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b02722 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 10, 2018
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
Investigation into the ash deposits in a coal-fired traveling grate boiler with the ammonia present in the flue gas Xiaolu Chen, Zhiyuan Liang*, Wenjun Yang, Qinxin Zhao (Key Laboratory of Thermo-Fluid Science and Engineering of MOE, School of Energy and Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China) ABSTRACT A study was undertaken of the ash deposits on different positions in a 140-MW coalfired traveling-grate boiler with the ammonia present in the flue gas. Several ash samples were collected from the slag tubes and three-stage heat exchanger of this boiler. The ash samples were examined using the combination of X-ray fluorescence (XRF) and X-ray diffraction (XRD) analysis to obtain their chemistry and mineralogy. Scanning electron microscopy and energy dispersive spectrum (SEM-EDS) analysis was conducted to determine the microstructure and compositions of the ash deposits. The results showed that the fouling of the slag tubes was mainly attributed to Fe2O3. At the three-stage heat exchanger, complicated sulfates played an important role in the deposition formation. Deposition mechanism of sulfates gradually changed with the gas temperature at different stages of the heat exchanger. NH4Al(SO4)2, Na3Fe(SO4)3 and KAl(SO4)2 were prone to deposit on the surface of heat exchanger than ammonium salts (e.g. (NH4)3H(SO4)2) at higher temperatures (>340 ℃). (NH4)3H(SO4)2 was found in the ash deposits at the outlet of the second and third stages of heat exchanger where the flue gas temperature was below 240 ℃. 1. Introduction
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Coal is the most important source of energy around the world, and this energy source would most likely last a long time. Coal consumption in China is especially high, exceeding 50% of the total amount of coal used globally 1-2. And the coal consumption is more than 60% of the total amount of energy consumption in China 3. Traveling-grate boilers have been widely used in the combustion system for solid fuels in China. There are about 480,000 coal-fired industrial boilers in China, of which 95% are travelinggrate boilers 4-5. Ash fouling is a common problem in coal-fired boilers, especially on the low-temperature heat transfer surface. It has attracted considerable attention over the last few decades. Several researchers have studied ash deposits on boiler components and associated devices in coal-fired power plants. Trace sulfur trioxide is formed in coal-fired boilers, resulting in the formation of sulfuric acid through the reaction of sulfur trioxide and water vapor. Sulfuric acid may condense on heat transfer surfaces, leading to corrosion and destruction of the surfaces 6. On the other hand, the chemical components of ash in coal are important factors affecting the ash deposition. Hueon Namkung et.al 7 found that the amounts of the Fe and Ca components were significantly related to the ash deposition trend in a real-time. Selective catalytic reduction (SCR) and selective non-catalytic reduction (SNCR) have been used for coal-fired power plants to remove oxides of nitrogen (NOx) in flue gas. As part of SCR and SNCR systems, ammonia (NH3) is injected into the flue gas. Most of the ammonia reacts with NOx, while some ammonia will continue through the system unreacted depending upon the process. This ammonia slip plays an important role in ash fouling if the ammonia combines with sulfuric acid and/or sulfur trioxide
ACS Paragon Plus Environment
Page 2 of 36
Page 3 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
present in the flue gas forming ammonium bisulfate (NH4HSO4) 8. NH4HSO4 is a kind of sticky and corrosive substance that forms on the metal elements of the air preheater in coal-fired power plants. However, post-combustion treatment strategies have not been widely used in coalfired traveling grate boilers. Few investigations were on the ash deposition formation in traveling-grate boilers, especially in the presence of ammonia in the flue gas. A study was undertaken of the ash deposits on different positions in a coal-fired traveling-grate boiler. Several ash samples were collected in the selected regions of the boiler. The primary goal of this work was to study the formation mechanisms of ash deposits at different positions of a coal-fired traveling-grate boiler with the ammonia present in the flue gas. 2. Sampling and analytical procedures Two days after boiler shutdown, several ash deposits were collected from different positions in a 140MW traveling grate boiler. A selective non-catalytic reduction (SNCR) denitrification system has been installed in this boiler for several months. The boiler has been in operation for about 2300 hours, and the excess air ratio (α) is 1.2. Previous work on the air preheater deposition of the boiler has been described in a separate paper 4
. The structure of the boiler has been shown in Figure 1, along with the exact locations
of ash samples. As seen the boiler has slag tubes and three-stage tubular heat exchanger. Eight ash samples were collected for analysis at different positions in the boiler. Samples A and B were collected at the top and bottom of the slag tubes, while samples C~H were sampled at the inlet and outlet of each stage exchanger. The flue gas
ACS Paragon Plus Environment
Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
temperature of the furnace outlet is about 720 ℃. The average flue gas temperatures at the three-stage heat exchanger outlets are 340 ℃, 237 ℃ and 140 ℃ respectively.
ST: Slag Tubes, 1st-HE: the first-stage heat exchanger, 2nd-HE: the second-stage heat exchanger, 3rd-HE: the third-stage heat exchanger Figure 1 Structure of the boiler and sampling positions The main flue gas parameters at the sampling positions are listed in Table 1. The average values of the parameters were calculated from the monitoring data during the boiler operating. Images of the ash samples are shown in Figure 2. Sampled from different positions of the slag tubes, samples A and B appeared dark-red: (A) The top of the slag tubes windward; (B) The bottom of the slag tubes leeward. Samples C, E, G were ash deposits on the tube surface of three-stage heat exchanger windward, while samples D, F, H were on the leeward side. As shown in Figure 2 (C, E, G), tube surfaces on the windward side were covered by thin and hard deposits. On the leeward side, the ash deposits (D, F, H) were thicker and looser than those windward. The ash deposits of the heat exchanger appeared gray color. Table 1 Main Flue Gas Parameters 4
ACS Paragon Plus Environment
Page 4 of 36
Page 5 of 36 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
Energy & Fuels
parameters
unit
average value
O2 SO2 NOx H2O Fly ash concentration
% mg/m3 mg/m3 % g/m3
6.5 1723.3 161.5 7.9 10.5
Figure 2 Images of the ash deposits All of the ash samples were ground to fine powders, preparing for XRF, XRD, and SEM-EDS analysis. A Bruker SP PIONEER X-ray fluorescence (XRF) instrument was used to carry out the element analysis of the ash samples. The X-ray diffraction experiments were conducted on a PANalytical X’pert MPD Pro instrument. The X-ray powder diffractogram was recorded at a scan rate of 2°/min in the range of 20°
